The thermal behavior of rice straw, microalgae, and their mixture was studied by thermogravimetric analysis (TGA). Co-combustion of rice straw and microalgae broadened the temperature range of combustion, facilitated ignition, and promoted burnout. The blend ratio of microalgae should be less than or equal to 10% in a 20%O-2/80%N-2 atmosphere and 30% in a 20%O-2/80%CO2 atmosphere to reach a higher comprehensive combustion index (CCI) value than the individual fuels. The co-combustion with a small ratio of microalgae could remedy partial negative effects on combustion performance caused by the replacement of N-2 using CO2. The interaction of blends depended on the atmosphere and temperature range. The prediction of the combustion performance of blends by a weighted sum of individual fuels worked better in an O-2/CO2 atmosphere at low temperatures, while better in an O-2/N-2 atmosphere at high temperatures. The simulation using the model which contained 2 parallel multi-order reactions matched with the thermogravimetric curves well, and blending reduced activation energies of the second stage.